According to a former Google employee, the system was under development as far back as 2006, and a second person with knowledge of the server says it was deployed sometime before the end of 2008.

The system is designed to simultaneously crunch various sets of stored data related to each individual user before deciding which online ads that user should see. Like the rest of Google's vast back-end infrastructure, it relies heavily on distributed-computing techniques. The task of serving up an ad is broken up into myriad pieces, and each piece is handled by a separate computing resource

One thread, for instance, might examine a user past search history. Another might look at past ad clicks. And so on. It operates in much the same way as, say, applications built atop Google App Engine, the Mountain View web service that lets you tap into the company's distributed infrastructure. With App Engine, a system request is terminated if it takes more than 30 seconds or returns more than 10MB of data. Everything must be broken into relatively small chucks that can then be distributed across the Google back-end.

In March 2009, Google announced that on YouTube and across its AdSense network of third-party sites, it had begun showing ads to websurfers based on the pages they've visited in the past. The company preferred to call this "interest-based advertising." But it's typically known as behavioral ad targeting.

"We think we can make online advertising even more relevant and useful by using additional information about the websites people visit," Google vice president Susan Wojcicki wrote at that time in a blog post entitled "Making ads more interesting."

"Today, we are launching 'interest-based' advertising as a beta test on our partner sites and on YouTube," she continued. "These ads will associate categories of interest - say sports, gardening, cars, pets - with your browser, based on the types of sites you visit and the pages you view. We may then use those interest categories to show you more relevant text and display ads."

Google called the new scheme a beta test and indicated it had not been deployed on its search engine. It did hint, however, that it had done at least a modicum of behavioral ad targeting before this. "To date, we have shown ads based mainly on what your interests are at a specific moment," Wolcicki said. The key word is "mainly."

In an effort to fend off privacy complaints, Google launched the scheme in tandem with something called an Ad Preferences Manager, which lets you view and edit the ad categories into which Google has placed you based on your past behavior. It also offered a cookie-based opt-out and a browser plug-in that kept your opt-out even when cookies were cleared.

Though Google did not discuss this during its official launch, it later told The Reg that the new scheme meant that Google was now using the same user cookie across both AdSense and its DoubleClick display ad platform. But it would not be drawn on how much user data was pooled across the two platforms.

We asked Google to discuss its Smart Ass server and its behavioral ad targeting schemes. But it has yet to respond. It's unclear whether behaviorial ad targeting occurs on services beyond YouTube and AdSense. And it's unclear what data - from what Google services - is fed into the Smart Ass system.

Clearly, Google's long-term goal is to serve ads that match the preferences and habits of individual users. This is why Google toys with the definition of "anonymization" when anonymizing your data history. It wants to know what you've done in the past so it can serve you ads accordingly.

This is the goal of Smart Ass - and it's significant step beyond the company's original ad serving system, which served ads only in response to search keywords or website text. The original system, says a former Google employee, is now known as Dumb Ass. ®